The subject invention relates to an improved proximity sensor having an improved orientation of a Hall-effect sensor in combination with a magnet for sensing the proximity of a workpiece.
Modern work transfer systems, such as high-speed transfer presses have required the movement of metal workpieces at high rates of speed. A holding device such as a robotic gripper, a suction cup, etc. disposed at the end of a robot arm, has been used to facilitate the movement of the workpieces into and out of the dies. It is desirable for a controller to detect if the holding device is actually gripping a metal workpiece.
A typical holding device having proximity sensing capabilities includes a proximity sensor. The proximity sensor comprises a current carrying coil in communication with a bridge circuit oscillator. As a metallic workpiece approaches the coil, the electrical current flowing through the coil is interrupted terminating oscillation in the bridge circuit. This generates a fault signal to a controller indicating a workpiece is proximate to the gripper. The absence of an interruption of electrical current when a workpiece should be present generates a fault signal.
Known proximity sensors utilizing a current carrying coil in communication with a bridge circuit oscillator for determination of proximity of a metal workpiece have not always proven sensitive enough. This type of sensor may not always detect the absence of a workpiece and, therefore, not generate a fault signal. An additional type of proximity sensor is a Hall-effect sensor axially aligned with the poles of a magnet. The Hall-effect sensor detects a magnetic field and generates a voltage output relative to the strength of the magnetic field detected. The proximity of metal to the magnet alters the magnetic field detected by the Hall-effect sensor resulting in an altered voltage output ultimately generating a fault signal. However, the axial orientation of the Hall-effect sensor to the poles of a magnet has not proven sensitive enough to provide consistently accurate workpiece detection.
Therefore, a need exists for a proximity sensor for sensing the proximity of a workpiece to a robotic gripper that is more sensitive and therefore more reliable than the currently utilized sensors.
A proximity sensing assembly for sensing the proximity of a metallic workpiece includes a holding device and a proximity sensor. The proximity sensor comprises a magnet and a Hall-effect sensor. The Hall-effect sensor is positioned in an offset relationship (i.e. non-concentric) to the poles of the magnet. A metallic workpiece alters the magnet field generated by the magnet. The Hall-effect sensor detects the presence or absence of a metallic workpiece by detecting a change in the magnetic field relative to the proximity of a workpiece and emitting an altered level of voltage. The voltage emitted by the Hall-effect sensor is proportionally related to the strength of the magnetic field.
A proximity sensor utilizing a magnet and a Hall-effect sensor positioned offset from the magnet poles has proven to be much more sensitive than known proximity sensors. The increase in sensitivity of the subject proximity sensor over known sensors reduces the probability of a workpiece detection error and, therefore, provides a more dependable robotic gripper for use in a transfer press operation.